Record Collapse of Earth's Upper Atmosphere Puzzles Scientists

An upper layer of Earth's atmosphere recently collapsed in an
unexpectedly large contraction, the sheer size of which has scientists
scratching their heads, NASA announced Thursday.

The layer of gas ? called the thermosphere ? is now
rebounding again. This type
of collapse is not rare, but its magnitude shocked scientists.

"This is the biggest contraction of the thermosphere in
at least 43 years," said John Emmert of the Naval Research Lab, lead
author of a paper announcing the finding in the June 19 issue of the journal Geophysical
Research Letters. "It's a Space Age record."

The collapse occurred during a period of relative solar
inactivity ? called a solar minimum from 2008 to 2009. These minimums are known
to cool and contract the thermosphere, however, the recent collapse was two to
three times greater than low solar
activity could explain.

"Something is going on that we do not understand,"
Emmert said.

The thermosphere lies high above the Earth's surface, close
to where our planet meets the edge of space. It ranges in altitude from 55
miles (90 km) to 370 miles (600 km) above the ground. At this height,
satellites and meteors fly and auroras shine. [Graphic:
Earth's Atmosphere Top to Bottom]

The thermosphere interacts strongly with the sun, so is very
affected by periods of high or low solar activity. This layer intercepts
extreme ultraviolet light (EUV) from the sun before it can reach the ground.

When solar activity is high, solar EUV warms the
thermosphere, causing it to puff up like a marshmallow held over a camp fire.
When solar activity is low, the opposite occurs.

Recently, solar activity has been at an
extreme low. In 2008 and 2009, sunspots were scarce, solar flares almost
non-existent, and solar EUV radiation was at a low ebb.

Still, the thermospheric collapse of 2008-2009 was not only
bigger than any previous collapse, it was also bigger than the sun's activity
alone could explain.

To calculate the collapse, Emmert analyzed the decay rates
of more than 5,000 satellites orbiting
above Earth between 1967 and 2010. This provided a space-time sampling of
thermospheric density, temperature, and pressure covering almost the entire
Space Age.

Emmert suggests carbon dioxide (CO2) in the thermosphere
might play a role in explaining the atmospheric collapse.

This gas acts as a coolant, shedding heat via infrared
radiation. It is widely-known that CO2 levels have been increasing in Earth's
atmosphere. Extra CO2 in the thermosphere could have magnified the cooling
action of solar minimum.

"But the numbers don't quite add up," Emmert said.
"Even when we take CO2 into account using our best understanding of how it
operates as a coolant, we cannot fully explain the thermosphere's
collapse."

The researchers hope further monitoring of the upper
atmosphere will help them get to the bottom of the situation.